Your body naturally contains small amounts of radioactive isotopes. Potassium-40 and carbon-14 are two examples that enter the body through ordinary food and the environment. Their unstable nuclei occasionally decay and emit radiation. This happens at a low level alongside background radiation from rocks, space and other natural sources. Radioactivity is therefore not something found only in nuclear power stations or laboratories. It is a natural part of the world around and inside us. Risk depends on the type of radiation, activity, dose and how exposure occurs, rather than on the word 'radioactive' alone.

The useful part of this physics is that it helps people predict and design real things. Engineers often begin with the same simplified model used in school, test where it works, then add more detail when measurements show that the real situation is more complicated.

Radioactive decay happens in unstable atomic nuclei and is random for any individual nucleus, although large samples follow predictable patterns. Alpha, beta and gamma radiation have different penetrating and ionising abilities, so their usefulness and risk depend on both type and exposure. Ionisation can damage cells, but carefully controlled radiation can also be aimed at unwanted tissue or used as a tracer because it is detectable in very small quantities.